The disclosed subject matter relates generally to gas discharge lasers, and more particularly, to methods and systems for increasing service life of a gas discharge laser chamber.
Electric discharge gas lasers are well known and have been available since soon after lasers were invented in the 1960s. A high voltage discharge between two electrodes excites a laser gas to produce a gaseous gain medium. A resonance cavity containing the gain medium permits stimulated amplification of light which is then extracted from the cavity in the form of a laser beam. Many of these electric discharge gas lasers are operated in a pulse mode.
Excimer lasers are one type of electric discharge gas laser. Excimer lasers have been known since the mid 1970s. A description of an excimer laser, useful for integrated circuit lithography, is described in U.S. Pat. No. 5,023,884 issued Jun. 11, 1991 entitled “Compact Excimer Laser.” The '884 patent has been assigned to Assignee of the present application. The '884 patent is hereby incorporated herein by reference for all purposes. The excimer laser described in '884 patent is a high repetition rate pulse laser though the laser disclosed there operated at about one third to one fourth of the pulse repetition rate required for contemporary laser systems used in photolithography.
These excimer lasers are typically used “around-the-clock” in an integrated circuit fabrication line in an integrated circuit lithography process. The integrated circuit lithography process produces many thousands of valuable integrated circuits per hour. Down-time in an integrated circuit fabrication line is very expensive. For this reason most of the components in an excimer laser are organized into modules which can be replaced within a few minutes and thereby minimize laser down time.
Electric discharge gas lasers of the type described in '884 patent utilize an electric pulse power system to produce the electrical discharges, between the two elongated electrodes. In such electric discharge gas laser systems, a direct current power supply charges a capacitor bank called a “charging capacitor” or “C0” to a predetermined and controlled voltage called the “charging voltage” for each pulse. The magnitude of this charging voltage may be in the range of about 800-1000 volts, and stepped up to about 16,000 volts (or greater). After C0 has been charged to the predetermined voltage, a solid state switch is closed allowing the electrical energy stored on C0 to pass through a series of magnetic pulse compression circuits and a step-up transformer to charge a so-called peaking capacitor Cp, which discharges very quickly across a pair of electrodes which produce the lasing discharge. Each discharge lasts about 20 to 50 ns.
Gas discharge laser chambers have a service life defined by several operational aspects. Two of the major service life limitations are the electrode life and the cleanliness of the laser window that passes the laser from the gas discharge chamber for use outside the gas discharge chamber. The pair of electrodes include a cathode and an anode. The electrodes are eroded by each of the electrical discharges across a discharge region in the space between the anode and the cathode. Each discharge ionizes the gas between the electrodes and erodes a portion of the material from one or both of the electrodes and releases the ions into the gas discharge laser chamber. The eroded material can combine with fluorine in the laser gas to form metal fluoride particles.
A mixture of gases is flowed through the discharge region to remove the ions from the discharge region before a subsequent discharge across the cathode to the anode (or vice versa). The particles can precipitate out of the gas flow and attach themselves onto the various inner surfaces of the gas discharge laser chamber.
A fan circulates the gases in the gas discharge laser chamber through the discharge region between the anode and the cathode. As the frequency of discharges increases the length of the time interval between each discharge is reduced. The fan must circulate the gases through the discharge region sufficiently to remove the ions and particles from the discharge region in each time interval. The laser should operate “arc-free”, that is without arcing between the high voltage electrode and grounded portions of the gas discharge laser chamber nearby through ions or particles nearby the inter-electrode discharge region which have not been sufficiently cleared from the discharge region by the circulating flow introduced by the circulating fan before the next subsequent electrical discharge. Therefore, as the time interval between each discharge is reduced the “arc-free velocity” of gases flowing through the discharge region must also increase in speed and/or efficiency, defining for each laser configuration and output pulse repetition rate an “arc-free fan speed” or “arc-free blower speed” of some definable RPM.
What is needed is to increase the service life of the gas discharge laser chamber is a longer life electrode and a more efficient removal of the ions and particles from the discharge region.